The present invention relates to improving the quality of products produced by plastic resin extrusion lines and has particular application to the production of blown film.
In the case of blown film, the plastic resin is extruded from a heated extruder having an annular die and the molten is pulled away along the die axis in the form of an expanded bubble. After the resin cools to a set diameter as a result of application of cooling air, the bubble is collapsed and passes into nip rolls for further manufacturing steps.
As the film is extruded, thickness variations occur about the circumference of the bubble. It is recognized that these variations are caused by such factors as circumferential nonuniformity in flow distribution channels (ports and spirals) within the die, melt viscosity nonuniformity, and inconsistent annular die gaps through which the polymer exits the die. Additionally, variability of the cooling air and non-uniformity of air aspirated into the cooling air stream from the atmosphere surrounding the extrusion line are major contributors to film thickness variation. Many film processors rely on conventional blown film equipment which typically yields an average variation of .+-.10 to 20% in film thickness, overall.
The presence of such thickness variations creates problems for downstream conversion equipment such as printing presses, laminators, or bag machines. In processes where the film is not converted in-line, but is wound onto a roll prior to converting, the thicker and thinner areas of many layers on the roll create hills and valleys on the roll surface which deform the film and magnifies the subsequent converting problem.
It is desired to obtain higher quality film during the extrusion process so that the downstream equipment can be run faster and produce better end use products with more consistent thickness.
The crudest and most widely practiced method for controlling blown film gauge variation is the use of fans and barriers placed strategically around the process to correct for ambient air variability. This is usually done in combination with manual operator adjustment of the annular die gap through which the polymer melt exits to help minimize die gap and melt viscosity variability effects. The main problem with this approach is that the ambient conditions surrounding the process constantly change and require continuous monitoring and barrier and/or fan repositioning. This approach also does nothing to take care of the relatively narrow thickness bands associated with the die ports and spirals. Use of this relatively primitive method results in typical thickness variations of .+-.10 to 15 percent.
Improvement over such manual adjustments is found in present art systems which actively measure the thickness of the film on-line. Employing closed loop control, these systems use computers which track thickness variation as it occurs in the still inflated bubble and calculate corrections to individual control zones within the die or cooling systems. These zones impart localized thickness variations which are opposite to those measured and thus to some extent correct for thickness deviations circumferentially around the bubble, including those caused by the ports and spirals within the die. Systems with control zones located within the die yield results in the range of .+-.4 to 7 percent thickness variability. Air cooling ring based systems do not historically perform as well and yield typically .+-.6 to 10 percent variability.
Though active systems as presently known do remove a significant portion of thickness variations, they unfortunately fall short of desired performance. Most film processors desire similar or better thickness tolerances to that achievable with films produced using cast film processes. Cast film is produced on flat, linear dies which are capable of direct, localized mechanical adjustment of the die gap with results typically of .+-.1 to 2 percent variation. There are several drawbacks which limit the overall performance of present art systems, and there are other drawbacks such as cost and complexity that deter their use.
A drawback that applies to both blown and cast film systems is that ultimate resolution of the thickness control is limited by the physical size of the actuator used to control a given zone. Typically, for this reason, active control zones are 1/2 to 1 inch wide. This allows for just 30 to 60 zones on moderately sized blown film dies of 10 to 12 inches in diameter. In a typical blown film process, thickness can change by several percent across the span of just one such zone. Resolution therefore limits present systems.
Another drawback, applicable to blown film lines, is reduced effective resolution due to expansion in diameter of the bubble outward from the region of control near the die lips. This expansion spreads the effect of each active control zone and thus reduces the ultimate circumferential resolution of the system. For instance, a 3 to 1 bubble expansion ratio with control zones of 1/2 inch width results in a final resolution after film expansion of 11/2 inch.
A major drawback of known active control system is high cost and complexity. Since each zone requires an actuator, individual control signals must be communicated and mounting hardware or mechanical linkages must be provided. Each actuator must be wired back to the central controller and as the number of zones is increased for better resolution, the complexity and associated cost rises in proportion. In retrofit applications, the existing die or air ring must also be replaced which adds significantly to the cost. Continuing operating costs associated with initial startup and maintenance of these complex systems are also quite high.
Yet another drawback is that present active systems do not adequately compensate for correlation problems caused by the location of the thickness measuring sensor. These sensors, by necessity, are located some distance away from the actual control zones and any shifts in the position of thickness bands will cause corrections to be applied in the wrong place. In order to average over time any remaining thickness variations on the finished roll, most blown film manufacturers rotate or oscillate either the die and cooling ring assembly or the nip roll and collapsing shield assembly. This oscillation or rotation causes the position of a given thickness band to shift in a spiral fashion, circumferentially as it travels from its point of origin to the sensing point. The amount of shift is related to many factors but is approximately proportional to the speed of oscillation or rotation and the time it takes for a point on the surface of the blown film bubble to travel from control zone to sensor. Presently known systems do attempt to correct for such correlation errors, however, a multitude of processing variables affect the position of thickness bands and in practice, it is very difficult to collect data and accurately calculate the exact position over time where the band of thickness originated. Resulting errors range from as little as 5 to as much as 20 degrees or higher and have served to further reduce the effective resolution of the prior systems.
Further drawbacks relate to individual approaches that have been proposed to control thickness variation. One approach seeks to control blown film thickness variation by direct mechanical adjustment and deformation of the die lips, similar to that used in cast processes. In these systems, localized, circumferentially variable, mechanical adjustments to the die lip cause detrimentally large hoop stress and elastic forces to develop in the round tube which resist deformation and spread the effect of the adjustment over a larger area than that intended thus limiting the effective resolution.
Another proposed approach utilizes direct, circumferentially variable, heating and/or cooling of the exit lip from the die. In these systems, individual controllers actuate cooling fluid flow through passages within the die lip or control spaced apart individual heaters embedded in the die lip which locally cool or heat the lips. Since heat spreads outward in all directions through the steel, the effect is not as locally concentrated as desired and resolution is reduced.
Yet another proposed approach employs circumferentially variable heating and/or cooling of the cooling fluid (typically air) which flows from the primary cooling ring surrounding the blown film bubble. Here again, individual actuators control the local temperature of the cooling fluid and affect the thickness of the film by changing the amount of local cooling. Due to the large volumes of air and associated turbulence involved, mixing takes place which significantly degrades the performance of this type of system. Also, since temperature differences of about 350 to 400 degrees fahrenheit exist between the polymer exiting the die and the air from the cooling ring, the effect on film thickness is very limited since it is difficult to locally cool an area by a significant amount. Heated systems can achieve larger cooling fluid temperature changes but have the drawback of losing cooling capacity since overall temperature is raised thus forcing the rate of film production to be lowered.
Another proposed approach alters in a circumferentially variable way, the flow of air exiting the primary cooling ring surrounding the exterior of the blown film bubble. Individual actuators mechanically alter the flow of cooling fluid through associated control zones. Thickness of the film is affected since more or less heat is removed due to the presence of more or less cooling fluid. Here again, significant mixing occurs which degrades the performance of these systems. Additionally, since cooling rings provide the major source of stabilization for the bubble and this stabilization is highly dependent on the flow of cooling fluid, these types of systems have an added drawback in that bubble stability is degraded and/or limitations must be placed on the range of adjustment of cooling fluid flow. This serves to greatly limit the performance of these systems.
In a case having superficial similarities to a blown film process but which in fact is significantly different, a processor has proposed use of variable internal cooling in the production of slit foam material. In this case a tube of extruding foam is pulled over a fixed annular mandrel which quenches the foamed material a fixed distance away from the die face. The cylindrical foam material is slit just after solidifying on the mandrel and is pulled away by a pair of nip rollers. The mandrel is supported from the outside by arms passing through the slits at the sides of the foam product. In this case the foam does not form a "bubble", since the inside is open to atmosphere. A thickness control air cooling ring of the type which circumferentially varies air flow described above, is mounted adjacent to the die on the exterior of the tubular foam. For this air ring, a multitude of flow control dampers with individual actuators are employed. A similar but mirror image air ring was proposed for use inside the open tubular form. For the internal air ring, all actuator control cables and air flow pipes were conveniently suspended from external supports which pass through the slit open sides of the once tubular form. Such arrangement can not function on a blown film production line because the external mounting structure cannot be used on tubular processes which have a closed, blown "bubble". Also cooling air flow changes which take place to control film thickness as described would cause a net flow imbalance of entrapped air that would cause final film size to be uncontrollable. This problem does not exist for the slit foam process since exhaust air simply flows either around or through holes in the mandrel and out the open sides of the slit tubular form.